Abstract Influenza infection or immunization elicit robust humoral responses but mutations that accumulate in the influenza virus hemagglutinin (HA) render both types of humoral responses seasonal: immunity against this season's flu does not reliably protect against next year's virus. Crucially, vaccine efficacy is influenced by each individual's history of infection and vaccination and this influence is long-lived and can be traced to the memory B cells compartments elicited by prior exposure to HA antigens. The immunological basis of this influence or clonal imprinting, is clonal dominance by memory B lymphocytes generated to cross-reactive HA antigens. Examples of clonal imprinting or original antigenic sin (OAS) have been well described, but the scope, durability, and structural constraints of imprinting have not been systematically studied. In project 2 we will use a novel method for large scale, single B-cell cloning to determine how and in which immunological compartments OAS is ?stored? and to define the structural similarities of epitope and paratope necessary for imprinting. Two types of memory B cells are responsible for the maintenance of humoral immune memory, IgM+ memory B cells (IgM+ Bmem) that have not undergone class-switch and class-switched (e.g., IgG+) Bmem; these small, quiescent lymphocytes reside in secondary lymphoid organs, where they are efficiently exposed to antigens. The population structure and dynamics of these populations following exposure to influenza hemagglutinins (HAs) is the focus of Project 2..